kernel: Remove idle thread cpu index on single-core devices

The idle thread got an index suffix in #23536 to make it easier to
identify different idle threads on different cores. This looks out of
place on single-core devices when the idle thread is listed next to
other kernel threads, such as main.

Remove the idle thread index on single-core platforms, and replace all
references to this format in tests and documentation.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>
3 files changed